1. Field of the Invention
The present invention concerns magnetic resonance guided high intensity focused ultrasound (MRgHIFU), and in particular concerns a method and an apparatus for focusing high intensity focused ultrasound (HIFU).
2. Description of the Prior Art
One challenge in MRgHIFU is to provide safe and thermally neutral focusing of the HIFU beam pattern using acoustic radiation force imaging (ARFI). The radiation force is localized and highly directional (along the main propagation axis of the HIFU beam) while negligible outside the focal zone. This force initiates a tissue displacement correlated to the amplitude of the acoustic field, and thus produces a phase shift that can be encoded in the MR signal using a motion encoding gradient (MEG) (1) Souchon et al. MRM 2008. (2) Plewes D. B. at al. JMRI 1995. (3) McDannold et al. MedPhys Med. Phys. 35(8):3748-58 (2008). In addition, ARFI also provides “stiffness weighted” images that may allow assessment of pre-therapy versus post-therapy changes in tissue. Since HIFU also causes tissue heating, temperature elevation and RFI effects are always associated, to various degrees.